1. Field of the Invention
Aluminum is generally used for an electrode wiring line of a semiconductor device in view of a resistance or workability. For the Al wiring line, a Cu-added Al alloy is generally used to prevent occurrence of an erroneous wiring line mode such as electro-migration or stress-migration, thereby improving reliability.
2. Description of the Related Art
The Al wiring line having a size of several .mu.m larger than the size of Al crystal generally has a point where three grain boundaries between the Al grains overlap, i.e., a triple point of the grain boundaries.
If a large current having, e.g., a current density of 10.sup.4 A/cm.sup.2 or more flows in the wiring line, Al atoms start to diffuse along the grain boundaries, so that the atoms are excess or deficient at the triple points of the grain boundaries to cause disconnection.
This phenomenon is called electro-migration. This is caused because the activation energy of the grain boundary of the Al-Cu alloy is about 1.4 eV while the activation energy of the grain boundary is about 0.6 eV, so that the grain boundary serves as a high-speed diffusion path.
Cu is added to the Al alloy to segregate Cu at the grain boundaries and prevent the Al atoms from diffusing along the grain boundaries. As shown in FIG. 1, the electro-migration life MTF [arb.unit] is prolonged depending on the content of Cu when the Cu concentration is about 8 wt % or less.
In a submicron region in which the wiring line size is 1 .mu.m or less and smaller than the crystal grain diameter, for example, as shown in FIG. 2, the above-described triple points disappear in the wiring line, leaving bamboo-node-like grain boundaries across the grain diameter boundaries. More specifically, as the wiring line size becomes larger, the number of triple points of the grain boundaries per unit length is increased, as indicated by a line L2, so that electro-migration tends to occur. On the other hand, as the wiring line size becomes smaller, the number of bamboo-node-like grain boundaries is increased, as indicated by a line L1. The electro-migration life at this time depends on the diffusion speed within the crystal grain, MTF is improved as a whole, and the wiring line life tends to be prolonged. The line L1 represents the number of bamboo-node-like grain boundaries per unit length with respect to the line size, and the line L2 represents the number of triple points of the grain boundaries per unit with respect to the line size.
In the wiring line having a size smaller than the grain diameter, as shown in FIG. 3, MTF is improved when the Cu concentration is about 0.1 wt % or less. If the Cu concentration exceeds 0.1 wt %, MTF is degraded.
That is because Cu is segregated at the grain boundaries when Cu is added exceeding the limit amount for dissolving Cu into Al crystal, and the diffusion speed in the longitudinal direction of the wiring line varies at the grain boundaries where Cu is segregated to cause disconnection.
However, the Cu concentration of the current wiring line is determined by the Cu concentration of a spattering target or vapor source, so that all the electrode wiring lines formed in the same process have the same Cu concentration. For this reason, the electrode wiring lines having the same Cu concentration are used although each electrode wiring line ranging from a wiring line having a size in the submicron region to be used as a signal line to a wiring line having a size of 10 .mu.m or more to be used as a power line has an optimal Cu concentration for its size. Thus, an optimal resistance to electro-migration for wiring lines having different sizes cannot be obtained.